A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix. A screw thread is one of the first standardized mechanical structure elements, which has been widely used in all kinds of fields. The screw thread can be divided into many species and types with the one defined on a cylinder being called a straight thread, the one defined on a cone being called a tapered thread, and the one defined on an end surface of a cylinder or a truncated cone being called a plane thread. The screw thread defined on an outer circular surface of a column-shaped body is called an external thread, the screw thread defined on an inner circular surface of a column-shaped body is called an internal thread, and the screw thread defined on an end surface of a column-shaped body is called an end thread. The screw thread with its rotation direction and helix orientation following the left hand grip rule is called a left-handed thread, and the screw thread with its rotation direction and helix orientation following the right hand grip rule is called a right-handed thread. The screw thread with a single helix on a cross section of a column-shaped body is called a single-start thread, the screw thread with double helixes is called a double-start thread, and the screw thread with multiple helixes is called a multi-start thread. The screw thread with a triangular cross section is called a triangular thread, the screw thread with a trapezoidal cross section is called a trapezoidal thread, the screw thread with a rectangular cross section is called a rectangular thread, and the screw thread with a zigzagging cross section is called a zigzagging thread. The screw thread used for fastening connection is called a fastening connection thread, the screw thread mainly used for transmission is called a transmission connection thread, and the screw thread used for conduit connection is called a conduit connection thread.
The screw thread of a fastener is the fastening connection thread, which is usually a cylinder thread with a triangle cross section, and is usually a right-handed thread while a left-handed thread is used for special use. The screw thread of a fastener is usually a single-start thread while a double-start or multi-start thread is used for celerity disassembly and assembly. The screw thread of a fastener comprises a screw thread pair, i.e. an internal thread and an external thread. The coupling nature of the screw thread pair is determining by the requirements of the connection system. The basic requirements of the fastener on the screw thread are loading-bearing and self-locking capability instead of transmission capability. The basic condition for self-locking thread is that an equivalent friction angle is no less than a lead angle. The value of an equivalent friction angle is related to a thread angle, the bigger the thread angle is, the bigger the equivalent friction angle is, and the more beneficial to self-locking that is the fundamental cause of the thread angle of fastener being 60° C. The wedge thread proposed in recent years, also called “Spiralock nut”, is an improvement to both of the metric screw threads and English screw threads, aiming at improving the self-locking capability of the screw thread defined on a fastener. The metric screw threads include ISO metric threads (M screw threads), aerospace metric threads (MJ screw threads), small metric threads (S screw threads) and Russian screw threads (MR screw threads), etc. the English screw threads include unified screw threads (UN screw threads), aerospace inch threads (UNJ screw threads), Whitworth threads and so on. In the English screw threads, the thread angle is 60° C. except that of the Whitworth thread.
The key factor for unthreading problem of the threaded fastener is the structure of the screw thread. In this case, the American engineers re-designed the geometric shape of the screw thread after researching on the shape the screw and force loaded on the fastener. At the end of 1970s, the technology of screw thread called “Spiralock” was proposed. The structure of “Spiralock” thread is that a wedge surface with a 30° C. inclination is defined at the root of a negative thread (internal triangular thread, i.e. female thread). When the bolt is coupled with the nut, a teeth portion of the external thread of a standard bolt abuts against the wedge incline of the internal thread, and hence generating interference screw locking to increase locking force. The increase of the locking force is due to the change of the thread angle which hence makes the angle between the normal force between the internal and external threads and an axis of the bolt to be 60° C. instead of 30° C., i.e. the angle of a standard thread. Obviously, the normal force of “Spiralock” thread is significantly greater than fasten stress. Thus, the locking friction generated is greatly increased. The technical level and professional direction are always focused on thread angles in researching on and solving the unthreading problem of threaded fasteners. The wedge threads are no exception. Moreover, the wedge threads are only partial variation of triangle threads. There is no wedge thread pair in use, and the wedge thread can be only used as female which couples with an external triangle thread. The locking force of the wedge thread is determined by the thread angle, and the loading force and self-locking capability of the wedge thread is the same as those of conventional thread techniques. The bolt or nut in the prior art has the defect of being easy unthreaded. With frequent shock of the device, the gap between the connection elements is increased, and the bolt and nut are hence unthreaded, or even fell out. In this case, it is easy for connectors in mechanical connection to depart from each other, or even cause security incident.
Aiming at the defects in the prior art, there has been a long-term exploration, and kinds of solutions have been proposed. For example, Chinese patent No. 201410521899.1 discloses a lock nut with changed thread pattern which comprises a nut body with an internal thread, a spiraling-up conic surface is defined on crest of the internal thread. As a preferred embodiment, an angle between a generatrix of the conic surface and center axis of the internal thread is 30° C. As a preferred embodiment, two ends of the nut body are an entrance and an exit of the bolt respectively. The entrance of the bolt extends outwardly to form a step, an inclination of a step surface of the step is S. an edge of the exit of the bolt is an inclined surface with an inclination of D. As an improvement of above preferred embodiment, the S is ranged from 20° C. to 30° C., and the D is 30° C.
Above solution an improvement to a certain extent in solving the problem of easy looseness of connection between fasteners in prior art. However, there are still the problems of low bonding strength, low self-locking capability and low loading capability in above solution.